NCDC added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.

U.S. Drought Highlights:

On the national scale, severe drought affected about seven percent of the contiguous United States as of the end of April 2001

National Overview

severe drought affected about seven percent of the contiguous United States as of the end of April 2001;

the total drought area has held steady at about 7 to 15 percent for the last seven months (see graph below left);

the coverage of the current drought peaked in August 2000 at about 36% of the contiguous U.S., which was as extensive as the major droughts of the last 40 years, but not as large as the "dust bowl" droughts of the 1930's and 1950's;

on a broad scale, the last two decades were characterized by unusual wetness with short periods of extensive droughts, whereas the 1930's and 1950's were characterized by prolonged periods of extensive droughts with little wetness (see graph below right);

although different parts of the U.S. have experienced unusually wet conditions during the last 19 months, there continues to be little change in the overall national wetness picture;

the percentage of the nation severely wet has held steady at about five to ten percent during this period (see graph below left).

Although some areas of the U.S. had well above normal precipitation, many areas were very dry. April was a dry month when precipitation is integrated across the nation. Eleven of the last 21 months have averaged well below the normal, while only two averaged well above normal (see graph to right). National 2001 precipitation ranks:

During April 2001, the U.S. Pacific Northwest continued to suffer from drought in spite of a wetter-than-normal month. Dry conditions returned to the southern Plains and the Northeast, and April marked the second dry month in a row for the Ohio Valley. This pattern is evident in:

The Keetch-Byram Drought Index (KBDI) is used by the National Interagency Fire Center to monitor the risk of wildfires. The KBDI showed a drying trend along the Gulf and South Atlantic coasts with an increasing threat of wildfires across Florida and in parts of the West by the end of April. The Forest Service fire danger analysis indicated a continuing risk of wildfires in the Southwestern U.S. and an increasing threat of wildfires in the northern High Plains and the southern Great Lakes.

Palmer Drought Indices

The Palmer Z Index shows how monthly moisture conditions depart from normal (short-term drought and wetness). April 2001 was severely dry across much of the country from the Northeast to Texas and Florida. Moist conditions during April brought temporary relief from the drought to the Pacific Northwest. Heavy rains from the central Plains to the northern Great Lakes contributed to flooding of the upper Mississippi River. The animated maps to the right show the geographical pattern of the moisture anomalies for the last 12 months. On these maps, the red shading denotes dry conditions while the green shading indicates wet conditions.

The Palmer Drought Index maps show long-term (cumulative) meteorological drought and wet conditions. Long-term meteorological drought persisted across the Pacific Northwest and intensified across Florida and from the southern Appalachians to Ohio Valley. The animated maps to the left show how the geographical pattern of the long-term (meteorological) moisture conditions has changed over the last 12 months. On these maps, the red shading denotes drought conditions while the green shading indicates wet conditions.

The Palmer Hydrological Drought Index maps show hydrological (long-term cumulative) drought and wet conditions, which more accurately reflect groundwater conditions, reservoir levels, etc. The PHDI indicates that hydrological drought persisted through the end of April across the Pacific Northwest states, and hydrologic conditions deteriorated across parts of the Southeast, Ohio Valley, and Northeast. The animated maps to the right show the geographical pattern of the long-term (hydrological) moisture anomalies for the last 12 months. On these maps, the red shading denotes dry conditions while the green shading indicates wet conditions.

Regional Drought Watch:

Standardized Precipitation Index

The Standardized Precipitation Index (SPI) is another way of measuring drought. The index is negative for drought, and positive for wet conditions. The SPI is a probability index that considers only precipitation, while Palmer's indices, shown in the previous maps, are water balance indices that consider water supply (precipitation), demand (evapotranspiration) and loss (runoff).

The seven maps below show the April 2001 spatial patterns of SPI for seven different periods ranging from one month (short-term conditions) to 24 months (long-term conditions). When taken together, they give a combined geographical and temporal picture of the severity of precipitation anomalies. The maps illustrate, for example:

short-term (1-month) dryness affected the Northeast, coastal Mid-Atlantic, Southeast, and southern Plains states but above-normal precipitation in March resulted in a neutral (mid-range) SPI in these regions for the 2-month period;

short-term (2-month) dryness was centered in the Ohio Valley and spread northward to the Great Lakes and south and westward to Tennessee and Oklahoma. The lack of precipitation was exacerbated by unusually warm April temperatures in this region;

March and April rains brought short-term relief to the Pacific Northwest, but longer-term (3-month to 24-month) drought still affects much of the Pacific Northwest and northern Rockies; and

the Southeast, Ohio Valley, and parts of the Northeast suffer from long-term drought.

Regional Overview

The following table shows the precipitation ranks for the nine standard U.S. regions for April 2001, the three-month period February-April 2001, and the twelve-month period May 2000-April 2001. A rank of 1 is driest and 107 (106 for May-April) is wettest.

The map to the right shows the departure from normal of the number of days with measurable precipitation for April 2001. The short-term dryness in the Northeast, Southeast, and southern Plains shows up on this map as significant below-normal (brown) number of precipitation days. The above-normal precipitation in the West and Northwest regions is reflected in a preponderance of above-normal rain days. The rain day anomaly pattern was mixed in Alaska and Hawaii.

Western U.S. Drought

April brought above-normal precipitation to much of the Pacific Northwest. This was a welcomed change from the unusually dry conditions that characterized much of the previous five months (see graph below left). Ranks for 2001:

second driest November-April (see graph below right), second only to the record drought of 1976-77.

The Governors of Washington, Oregon, and Idaho declared drought emergencies in their states. The extreme dryness of this year's rainy season caused the statewide Palmer Drought Index in Oregon to rapidly decrease, reaching its most severe level since 1977.

Parts of the northern Rockies and northern High Plains have experienced dry conditions. For example, Montana had below-average surface wetness and soil moisture in April. Long-term dry conditions have resulted in the 13th driest May-April on record with the statewide Palmer Drought Index dropping to the lowest levels since the drought of the late 1980's.

The West region (California and Nevada) had a wet April. The 2000-2001 rainy season started out dry (see graph to left), but rains during the last half helped replenish reservoirs and add to the mountain snowpack. The severity of the dryness at the start of the season, however, resulted in the 29th driest May-April on record.

Southeast and Southern Plains Drought

April was dry for much of the Southeast and southern Plains regions (see graphs below). Although some parts of the Southeast have seen recent drought relief, long-term drought continued region-wide. The hardest-hit areas have had moisture deficits for the last three years, tracing the drought to the onset of La Niña conditions in early 1998. For the Southeast region,

nine of the last 12 months averaged below to much below normal (see graph below left),

April 2001 ranked as the fourth driest April on record for the southern Plains. This is a departure from the last three months, which were wetter than normal (see graph above right). When the unusual wetness of October 2000-March 2001 is combined with the dryness of April this year and last summer, the precipitation ranks fall in the mid-range (see regional overview table above).

Central Region Dryness

April marked the second month in a row with much below normal precipitation averaged across the Central region (see graph below left). Ranks for 2001:

Northeast Region Dryness

April 2001 was the third driest April on record in the Northeast region (see graph below right) and marked a return to the predominantly dry conditions which have characterized the region during much of the last seven months (see graph below left). October 2000-April 2001 ranked as the ninth driest such period in the 1895-2001 record, but when the unusual wetness of late spring and early summer 2000 is factored in, the last 12 months ranked as only the 40th driest May-April.

The heavy snow cover at the beginning of the month rapidly melted and was nearly gone by late April. Unusually warm temperatures during late April and early May increased the threat of fire danger to parts of the region, especially Maine, which had the driest April and seventh driest February-April.